#include <iostream>
#include <algorithm>
using namespace std;
using namespace std;
long int n,k,sum[10001],num = 1,f[10001];
struct ren{
    long int ks,js;
};
ren z[10001];
int cmp(ren a,ren b){
    return a.ks > b.ks;
}
int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    long int i,j;
    cin>>n>>k;
    for(i=1;i<=k;i++)
    {
        cin>>z[i].ks>>z[i].js;
        sum[z[i].ks]++;
    }
    sort(z+1,z+k+1,cmp);
    for(i=n;i>=1;i--)
    {
        if(sum[i]==0)
            f[i]=f[i+1]+1;
        else for(j=1;j<=sum[i];j++)
            {
                if(f[i+z[num].js]>f[i])
                    f[i]=f[i+z[num].js];
                num++;
            }
    }
    cout<<f[1]<<endl;
    return 0;
}
